The phylogenetic tree show it.
https://botanistinthekitchen.files.wordpress.com/2013/04/201...
I don't think Indian cooking uses a lot of brassicas as greens. Lots of spinach, plus amaranth, also a Caryophyllale, and fenugreek/methi, a Fabale. Mustard leaf is a brassica, though.
Not a botanist, but I think it may be considered as a green for practical purposes. Looked up Wikipedia. Technically the edible part is a flower or such. But is sold in greengrocer* shops alongside other greens of all sorts.
*British term, produce shops may be the US eqiivalent.
Just learned about this one today. Purslane https://en.wikipedia.org/wiki/Portulaca_oleracea
They aren't bad, per se, but have a very short season and are really more of a "winter stores have run low and too early to harvest anything, but I am really hungry..." situation.
Edit: The article seems to refer that grape leaves are edible but it also says that grape leaves were not included in this article on purpose.
Even so, you usually don't base a whole dish around them the way you can with other leafy greens. The tannic bite easily becomes too much.